1. Field of the Invention
The present invention relates to an image processing technique and, more particularly, to a keystone correction processing technique for correcting keystone distortion in a front projector or the like.
2. Description of the Related Art
As a technique of electronically correcting trapezoidal distortion of an image which occurs when a liquid crystal projector projects the image on a screen, an electronic trapezoidal distortion correction processing technique has been conventionally known (Japanese Patent Laid-Open No. 2009-44488).
The electronic trapezoidal distortion correction processing technique deforms the shape of an image so as to cancel out trapezoidal distortion to project the resultant image as an image having the same shape as that of the original image. Performing interpolation/resampling processing will generate an image corresponding to the deformed shape.
The electronic trapezoidal distortion correction processing technique reduces an image when deforming its shape. With a reduction in image, high-frequency components of the spatial frequency components of the image become aliasing noise, resulting in image deterioration such as moire.
To reduce such deterioration, filter processing is performed. However, a conventional technique performs filter processing by using a filter having a predetermined cutoff frequency independently of the positions of the pixels in an image before shape deformation processing.
After the shape deformation, the distances between adjacent pixels change, and the cutoff frequency in filter processing changes. For this reason, in various regions in the image after the shape deformation, the cutoff frequency in the filter processing does not coincide with the Nyquist frequency in resampling of the image after the shape deformation. For this reason, the conventional technique has a problem that an image excessively blurs and moire remains, resulting in a deterioration in image quality.